Signal transport time based positioning methods may be used when geographically positioning UEs in a wireless communication network. Examples of such signal transport time based positioning systems are Time of Arrival, TOA-based or Time Difference of Arrival, TDOA-based positioning methods. The signal transport time based positioning methods are generally based on one of the two following principles: One transmitting unit transmits a radio signal that is received by a plurality of receiving units, or a plurality of transmitting units transmit simultaneously or with a known time difference a signal that is received by one receiving unit. In the first principle, the transmitting unit may be a UE which is to be positioned and the plurality of receiving units may be base stations, e.g. eNodeBs. In the second principle, the plurality of transmitting units may be base stations and the receiving unit may be a UE that is to be positioned. Measurements are then performed to detect receiving time (aka. time point) and possibly also transmitting time. Based on the detected times, signal transport times indicating distance between receiving and sending units may be determined.
In the TOA-based method, the time when sending of a radio signal from a transmitting unit is detected and the time of arrival, TOA, when the signal arrives at the receiving unit is detected. A network node such as a positioning node receives the detected times from the respective nodes and determines a time difference that would be a measure of the time it took for the signal to travel between the transmitting node and the receiving node and therefore a measure of the signal path distance.
In the TDOA-based method, such as the Observed TDOA, OTDOA, used in Long Term Evolution, LTE-based networks, a plurality of eNodeBs transmit Positioning Reference Signals (PRS). The UE performs measurements on the signals and estimates the TOAs of the different signals. From the TOA values the UE calculates Reference Signal Time Difference, RSTD, values, which is the difference between TOA pairs. The RSTD values are then reported to the positioning node that performs the TDOA positioning based on the different TOA pairs.
When determining the position of UEs in a wireless communication network using signal transport time based positioning methods, the positions of the base station antenna(s) sending/receiving the signals used for the determination are used as reference points. If there is an error in the antenna position, i.e. if there is a difference between the real antenna position and the coordinate value specifying the antenna position, this will lead to an error in the UE position estimated based on the TOA/TDOA values and the coordinate value specifying the base station antenna position. Another parameter that may cause errors in the estimated TOA/TDOA values and consequently in the UE position determination is if there is an unknown delay in the signal path. If for example the transmitter thinks it transmits the signal at time 0 ns, but the signal instead actually leaves the antenna X ns later. The TOA estimation will also in this case contain an error due to the unknown delay in the signal path.
FIGS. 1a and 1b show a system where these two errors/mismatches between reality and the system view, i.e. internal coordinates and settings, are illustrated. FIG. 1a illustrates the system view of a base station node 120. The system expects an antenna 121 of the base station 120 to be at position P and that the delay between a transmitter TX of the base station 120 and the antenna 121 is D. FIG. 1b shows how the system is in reality. The antenna 121 is not in position P but in position P+ΔP and the delay is not exactly D but D+ΔD.
If the ΔP and possible also ΔD could be estimated the system/nodes could be updated with these parameters and thereby reduce the error on the estimated positions for the UEs, estimated based on TOA/TDOA values and base station antenna coordinates.